Conventional control circuits for d.c. motors are utilized in many different applications such as paper processing apparatus, loaders, trucks, exercise equipment such as treadmills, and the like. These known circuits usually employ silicon controlled rectifiers integrated in the motor electric supply circuit, and some device to produce a train of pulses arranged to effect periodic conduction of the rectifiers. In this manner, the motor is energized by pulses of direct current, and control of the duration of the pulses is utilized for motor control purposes such as speed of motor drive, and for compensation during various load conditions.
Such control circuits also are known to provide compensation for the effect of the internal resistance in the motor armature. One such control circuit is described in U.S. Pat. No. 4,839,570, ('570 patent), which patent is owned by the assignee of the instant invention. The invention described and claimed herein is an improvement to the invention described and claimed in the '570 patent.
The control circuit described in the '570 patent provides compensation for motor armature voltage loss by generating a sensing signal, indicative of the current through a sensing resistor, to sense the voltage drop across the armature of the controlled d.c. motor. The sensing signal is conducted to an amplifier whereat it is amplified and compared to a reference voltage to produce an error signal in accordance with current limiting need for the motor. This amplifier is dedicated solely to the current limiting function, and the amplified corrective signal is fed directly into a trigger circuit, which controls the d.c. power supply to the motor. It was the object of the invention in the '570 patent to control the current supply to a d.c. motor so that the circuit response time was extremely fast and its operative capability accurate and predictable.
This was accomplished by supplying a corrective signal to a d.c. motor for current limiting purposes in the shortest period of time, by producing the signal in a circuit dedicated solely for this purpose, and by directly feeding the signal into the power supply to the motor. The teachings of the '570 patent are specifically incorporated herein by reference.
Although the control circuit described and claimed in the '570 patent had many advantageous features, it failed to provide line dropout protection. Providing line dropout protection in a d.c. motor is an important consideration in many applications, where line dropout protection is defined as preventing motor restart unless the motor speed control is first returned to a zero setting. For example, in a piece of equipment such as an exercise treadmill, power failure during operation is not dangerous as the moving belt on the treadmill will slowly stop. However, should the treadmill suddenly re-start, with the speed control set in a high speed position, the user of the equipment could be thrown from the treadmill, with the possibility of sustaining serious injury. Similar considerations are, of course, present in any piece of machinery, or equipment, powered by a d.c. motor, wherein an unexpected high speed start could damage both equipment and operating personnel.
It is, therefore, an object of the instant invention to provide a motor control circuit for a d.c. motor, which provides line dropout protection.
It is a further object of one embodiment of the instant invention to provide a line dropout circuit for a d.c. motor, which will prevent operation of the motor when power is applied, unless the main speed control is in a zero position.
It is another object of a second embodiment of the instant invention to provide a line dropout circuit for a d.c. motor, which will prevent excessive acceleration of the motor when power is applied.
It is a still further and general object of the instant invention to provide an inexpensive, yet highly reliable, d.c. motor control circuit with line dropout protection.